The present invention relates to a friction material for a dry friction device, a method of making such a dry friction material, and a dry friction device equipped with such a material, and the like.
More particularly, the dry friction device may, without limitation, be a clutch disc or a brake disc, for example for a motor vehicle, and the friction material may be formed into a friction liner in the form of a flat annular ring.
Dry friction materials employed at the present time fall into three main types, namely, filament based materials, woven materials, and molded materials.
Filament based materials and woven materials, consist essentially of filaments which are provided for reinforcing purposes, being impregnated with thermosetting resin or with rubber, and having various fillers incorporated. Materials of this general type have various disadvantages. Their manufacture calls for the use of noxious solvents, and special arrangements have to be made in order to prevent such solvents escaping into the surrounding atmosphere.
It has been proposed to replace these noxious solvents by water, but in that case it is necessary to provide means for purifying the water, or for purifying vapour that is rejected at the end of the manufacturing process.
These arrangements, which not only add complication and difficulty to the methods of manufacture, are furthermore not wholly effective.
As to dry friction materials of the molded type, these are generally made in a humid vacuum, in a mixing process which again has the same type of disadvantage.
In addition, in the methods currently used, only reinforcing fibers of relatively short fiber length can be employed. In general, the mean fiber length is less than 5 mm. As a result, these molded friction materials are not strong enough to resist the effects of centrifugal force, in particular when they are hot; and this makes them unsuitable for use, for example, in a dry clutch friction disc which is driven in rotation at speeds that may exceed 6000 revolutions per minute.
An object of the present invention is to overcome the above mentioned drawbacks, by proposing a dry friction material which has improved technical characteristics, while at the same time having a selling price which is less than that of known materials.
Another object of the invention is to provide such a material in which the method of making the material does not involve pollution of the natural environment, but also enables used friction materials, and the residues resulting from machining of the new friction materials, to be recycled.
A further object of the invention is to provide a dry friction material which has a stable coefficient of friction combined with high resistance to wear.
According to the invention in a first aspect, a friction material for equipping an apparatus employing dry friction, and more particularly a clutch or brake disc, is characterised in that it consists of a mat of fibers impregnated with a thermosetting resin, and in that the fibers have a length at least equal to 40 mm.
The fibers have a mean length preferably at least equal to 120 mm.
Preferably, the fibers are selected from the group consisting of fibers of cotton, viscose, linen, polyacrylonitrile, preoxidised polyacrylonitrile, para-aramide, meta-aramide, mineral fibers such as E-C-R glass, rock wool, and ceramic.
According to a preferred feature of the invention, glass is incorporated in the mat in the form of continuous filament of glass yarn comprising fibers of small diameter, and/or in the form of roving a glass yarn with more larger fibers, and/or in textured or bulked form.
The diameters of the glass fibers are preferably in the range between 6 and 21 micrometres.
Preferably, the glass fibers have been subjected to adhesion, impregnation of phenolic resin, and/or impregnation of rubber.
Powder fillers are preferably incorporated in the mat. Preferably, the powder fillers comprise, wholly or partly, at least one substance selected from the following elements, and compounds: copper, rock wool, nitryl rubber powder, carbon black, hexamethylene tectramine, gilsonite, litharge, melamine resin, phenolic resin, sulphur, cardolite, zirconium silicate, iron sulphide, alumina, latex, used clutch liners reduced to powder, friction liner rectification dust, zinc oxide.
Short fibrous fillers may be incorporated in the mat or be present in the latter.
According to the invention in a second aspect, a method of making a friction liner for dry friction operation is characterised by the following steps:
(a) a mixture of fibers of the same nature or of different materials, as set forth above in relation to the invention in its first aspect, is made in a mixer;
(b) the mixture is carded so as to form a carded web;
(c) the carded web is napped;
(d) glass is incorporated in the carded web at the instant of the napping step;
(e) the mat is subjected to a preliminary rolling treatment, or condensed.
Powder fillers are preferably sprinkled on the carded web between steps (b) and (c).
The carding operation is preferably carried out using a card of the wool carding type.
According to a preferred feature of the invention in its second aspect, the mat is brought to a thickness in the range between 3 mm and 10 mm, between heating platens. This step is preferably carried out at a temperature between 50xc2x0 C. and 100xc2x0 C. and at a pressure which is preferably between 1 bar and 10 bars.
The mat is then preferably cut into bands across its width, these cut bands preferably having a width in the range between 100 mm and 450 mm. The cut bands are then preferably welded together, preferably with the use of thin heating plates, the temperature of which is preferably in the range between 50xc2x0 C. and 100xc2x0 C. The cut bands are preferably welded together with an overlap between them in the range between 3 mm and 20 mm.
This produces a continuous welded band or strip, which is preferably then wound on to a winding member such as a bobbin, after which it is drawn off the latter and formed into a tube having an internal diameter which is defined by a mandrel having a variable diameter. The external diameter of this tube is preferably obtained by means of an appropriate draw length, i.e. by appropriate control of the rate at which the welded band is drawn from,the bobbin to the mandrel. The internal and external diameters of this tube preferably lie in the range between 100 mm and 250 mm, and 140 mm and 450 mm respectively. Winding of the welded band on to the mandrel is preferably carried out in contact with heating cylinders.
After the tube has been formed, an annular ring can be cut from the tube. The depth of this annular ring preferably lies in the range between 8 mm and 25 mm, and the step of cutting the annular ring from the tube is preferably effected by means of rotary slitters.
The annular ring is preferably placed in the bottom of a mold in which it is subjected to heat treatment under pressure, at a temperature which is preferably in the range 150xc2x0 C. to 250xc2x0 C., with a pressure being applied to the material that preferably varies between 20 bars and 300 bars.
According to the invention in a third aspect, a rotatable clutch friction disc, for dry friction operation, includes at least one friction liner, and preferably two friction liners disposed on either side of a support member, and is characterised in that the friction liner is made of a friction material in accordance with the invention in its first aspect. This material is preferably made by a method according to the second aspect of the invention.